The release liners for use in pressure-sensitive adhesive tapes are generally constituted of a release-liner substrate and a release agent layer formed on the substrate. Known as the release agent layer is one formed by applying a silicone release agent and curing it. For example, in double-sided pressure-sensitive adhesive tapes or the like, the release liners coated with a silicone release agent and having, formed thereon, a pressure-sensitive adhesive layer comprising an acrylic pressure-sensitive adhesive are included in examples of that release liner. However, such a release liner coated with a silicone release agent has a problem that when the pressure-sensitive adhesive tape is used, part of the silicone compound contained in the release liner remains adherent to the adhesive layer side and thus contaminates the adhesive layer, resulting in considerably impaired adhesive performance. In addition, use of this kind of pressure-sensitive adhesive tape for fixing or other properties in producing electronic appliances such as HDDs (magnetic recording apparatus), especially in inner parts of such electronic appliances, has a problem that the pressure-sensitive adhesive tape causes the corrosion of inner parts of the electronic appliances and operation errors. This is because those parts of the adhesive layer which have been contaminated with the silicone compound contained in the release liner serve as a siloxane gas source.
On the other hand, release liners to which a releasing function has been imparted without using a silicone release agent such as that described above are known. Examples thereof include: ones comprising a release-liner substrate and a release layer which comprises a low-density polyethylene resin layer and has been formed on the substrate by extrusion laminating while inhibiting the layer from undergoing surface oxidation (see JP-B-51-20205 (The term “JP-B” as used herein means an “examined Japanese patent application”) and JP-A-U-63-85642 (the term “JP-A-U” as used herein means an “unexamined published Japanese utility model application”)); ones comprising a release-liner substrate and a release layer formed thereon by extrusion laminating which comprises a resin blend of low-density polyethylene with a lowly crystalline ethylene/propylene copolymer or lowly crystalline ethylene/butene-1 random copolymer (see JP-B-57-45790 and JP-A-6-155687 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)); and one which comprises a substrate and an ethylene polymer release layer formed in a specific thickness ratio over the substrate through an undercoat layer and which has a specific value of outgassing amount (see JP-A-2003-127299). Also known is a technique in which a pressure-sensitive adhesive is caused to conform to surface irregularities of a release liner and thereby form recesses and protrusions on the surface of the pressure-sensitive adhesive layer for the purpose of imparting a fine structure to the pressure-sensitive adhesive layer (see JP-T-9-50423 and JP-T-2001-507732 (The term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”). Furthermore, release liners employing a fluorochemical release agent as another release agent are widely known. These release liners are used as pressure-sensitive adhesive sheets or tapes after a pressure-sensitive adhesive layer is formed thereon in the subsequent step.
Pressure-sensitive adhesive tapes (or sheets) for hard-disk drives (HDDS) are applied by automatic machine application, which necessitates no personnel cost, in order to cope with the recent rapid cost reduction in hard-disk drives and are applied at a high speed in order to further increase the efficiency of mass production. In this high-speed automatic machine application, an operation is first conducted which comprises holding a pressure-sensitive adhesive tape on an air suction table by air suction on the base side (the side opposite to the pressure-sensitive adhesive layer, or the back side) of the tape, stripping the release liner from the tape thus held, and applying the tape to a hard-disk drive. In this operation, in case where the air suction is too strong, not only the pressure-sensitive adhesive tape itself becomes rugged, resulting in a considerably impaired appearance, but also airtightness, which is one of the functions of pressure-sensitive adhesive tapes for hard-disk drives (HDDs), is impaired. Because of this, air suction can be conducted only when the power of air suction is on the minimum necessary level. Consequently, in such high-speed stripping, the peel force necessary for stripping the release liner from the pressure-sensitive adhesive tape (or sheet) for hard-disk drives (HDDs) is required to be lower than the minimum necessary force of air suction. This peel force lower than the air suction force is 0.3 N/50 mm or lower in terms of the peel force of the release liner stripped under the conditions of a peel rate of 1 m/min and a peel angle of 180°.
Incidentally, a release liner is known in which the release layer has surface irregularities so as to reduce peel force (see JP-A-2002-219778). However, the release agent employed in this related-art release liner having surface irregularities is not a polyolefin resin but a silicone resin, fluororesin, or the like.
Furthermore, precision electronic appliances including HDDs (magnetic recording apparatus) are susceptible to static electricity. In case where static electricity generates in an HDD production step, there is the possibility that troubles might be aroused in the appliances by the static electricity.
However, among the release liners to which a releasing function has been imparted without using a silicone release agent, the release liners having a release layer comprising a polyolefin resin, for example, do not show a satisfactory releasing function in application to pressure-sensitive adhesives having high tackiness and necessitate a peel force higher than the air suction force. These release liners are not usable in high-speed stripping by an automatic machine. Furthermore, part of the pressure-sensitive adhesive is transferred to the release layer upon stripping or the stripping operation results in pulse stripping (so-called “stick slip”). Consequently, the pressure-sensitive adhesive layer comes to have a rough surface and, hence, the performance originally possessed by the pressure-sensitive adhesive layer cannot be effectively exhibited.
On the other hand, the release liners employing a fluorochemical release agent are satisfactory with respect to smooth stripping from the pressure-sensitive adhesive layer. However, because these release liners are highly expensive, use thereof makes it impossible to cope with the demand for material cost reduction which accompanies the recent trend toward cost reduction in hard-disk drives.
Furthermore, when general pressure-sensitive adhesive tapes or sheets are used, there are cases where stripping the release liner causes static buildup. There has been the possibility that a minute current which flows in this case might arouse troubles in precision electronic appliances.